Solvent extraction of lube distillates and de-asphalted oils with furfural or N-methyl pyrrolidone (NMP) is utilized to remove the 2+ ring aromatics and heteroatoms, resulting in improved thermal and oxidation stability of lubricant basestocks. The aromatic-rich lube extract "by products" from the solvent extraction process, such as furfural extracts, derived from vacuum distillates or vacuum resids possess unique solvency properties that make them ideal as process oils for rubber and ink oil manufacture.
While bright stock or residual aromatic extracts derived from vacuum residuals are typically non-carcinogenic, solvent extracts derived from neutral distillates are among the more carcinogenic products produced in the refining of petroleum.
Recently, there has been growing concern over public and worker exposure to the polynuclear aromatics (PNA's) from distillate aromatic extracts (DAE's) used in the tire industry. Untreated lube extracts derived from vacuum distillates have been demonstrated to produce a number of tumors in mouse skin painting bioassays, and as such they are labeled "May Cause Cancer" in the European Union.
The mutagenicity of lube extracts is believed to be a function of the 3-7 ring polynuclear aromatic content in the extract. Due to concerns for worker exposure to these carcinogenic extracts, public exposure to road-side tire dust and used tires, the European tire industry is interested in converting from using the currently available toxic DAE's to non-toxic DAE's.
Since petroleum refiners that market these products must provide labels outlining potential risks associated with the use of these products, there is a significant incentive to upgrade DAE's to make them non-carcinogenic.
The EU utilizes the polycyclic aromatic (PCA) content of DAE's as an indication of their toxicity, as measured by a gravimetric test, IP346. For treated DAE's the EU requires the PCA content of the product as measured by IP346 to be below 3 weight % for non-toxic labeling.
The mutagenicity of petroleum distillates may also be measured on a Mutagenicity Index (M.I.) scale via an ASTM-approved procedure called the Modified Ames Assay, as described in "Predicting Carcinogenicity of Petroleum Distillation Fractions using a Modified Salmonella Mutagenicity Assay", by G. R. Blackburn, Cell Biology and Toxicology, 2, pp. 63-84, 1986 and U.S. Pat. No. 4,499,187, the entire contents of which are hereby incorporated by reference. Current policy in the U.S. is that the measured M.I. must be less than 1 for non-toxic labeling.
As will be evident from the following examples, a PCA content of 3 wt %, according to IP346, does not necessarily equate to a M.I. of 1. It should be noted that the EU requirement is a regulatory one, while the M.I. is based on an empirical evaluation of mutagenicity of samples.
Conventional vacuum stripping of DAE's has been demonstrated to be ineffective in reducing PCA content below 3 wt %, since the boiling points of many of the PCA's fall within the same range as that of the desireable components of the process oils to be produced. Likewise, oxidation of PCA's to reduce toxicity has been shown to be of limited effect. While some reduction in M.I. can be obtained by oxidation, the reduction is insufficient to bring the products within the non-toxic range.
One method of treating process oils to reduce the PCA content is described in EP 0 417 980 A1, wherein process oils with an aromatic content of more than 50 wt % and a PCA content of less than 3 wt % are prepared from a primary extract of a lubricating oil distillate by re-extracting with a polar solvent in a counter-current extraction column, such that the volume ratio of the primary extract to the polar solvent is in the range of from 1:1 to 1:1.8. Notably, the polar solvent used for the re-extraction is the same solvent utilized in the initial extraction step.
Disadvantageously, according to EP 0 417 980 A1, the temperature in the head region of the extraction column must be at least 10.degree. C. higher than the temperature at the bottom of the column, requiring careful monitoring and control of column temperature differentials.
Therefore, it would be desirable to develop a method of treating lubricating oil extracts to reduce the PCA content below 3 wt %, without expensive equipment for temperature monitoring and control of a counter-current extraction column.